Field of the Invention
The present invention relates to bonding wires for high-speed signal lines that are used for connecting pad electrodes of semiconductor devices and lead electrodes on circuit boards and, more particularly, to a bonding wire for a high-speed signal line using a frequency of 1 to 15 GHz.
Description of the Related Art
Recently, with the development of manufacturing technology of semiconductor devices, it has become increasingly common to mount, in cellular phones and the like, semiconductor integrated circuit devices for high-speed signal lines using super-high frequencies exceeding a several GHz band. Conventionally, high-purity gold bonding wires with purity of 99.99 mass % or more have been commonly used for high-frequency transmission. However, when the normal bonding wire is used to connect the semiconductor device for the high-speed signal line using the super-high frequency and an wiring electrode and the like, in a several GHz to over 10 GHz band, high-frequency resistance by a super-high frequency signal increases further since the super-high frequency signal flows through a skin layer of the bonding wire. For this reason, receiving sensitivity, transmission output and the like are reduced when the high-purity gold bonding wire is used.
Therefore, examination of a high-purity silver (Ag) wire having purity of 99.99% and having an electric specific resistance value of 1.6 μΩcm and the like was made, as compared with high-purity gold (Au) having an electric specific resistance value of 2.4 μΩcm. However, bulk high-purity silver (Ag) is too soft to undergo a manufacturing process of the bonding wire, that is, melting, casting, cleaning, and continuous wire drawing. Further, silver (Ag) is sulfurized in the atmosphere, and a silver sulfide (Ag2S) film is formed on the skin layer of the bonding wire. For this reason, the pure silver bonding wire has not been used as it is for the high-frequency transmission, in which the signal flows through the skin layer having the thickness of several μm, and the pure silver bonding wire is made suitable for practical use only after pure gold plating, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-59963. With the pure silver bonding wire, strong silver sulfide (Ag2S) hardens a molten ball at the time of forming the molten ball to be used for ball bonding, and therefore, it is not possible to form the stable molten ball, before being used as the high-speed signal line using the super-high frequency.
Meanwhile, a bonding wire of an Ag—Au binary alloy, in which 10000 to 55000 mass ppm of Au and 1 to 100 mass ppm of Bi are added to Ag, and further, a bonding wire of an Ag—Au—Pd ternary alloy, in which 20000 mass ppm or less of Pd is added to the above-described wire, have been developed (Japanese Unexamined Patent Application Publication No. 2012-49198), for the purpose of preventing electric specific resistance of Ag, having high electroconductivity, from being reduced substantially, and allowing the electric specific resistance of Ag to be 3.1 μΩcm or less, which is the same level as that of the Au wire that has purity of 99% and that is widely used at present. Here, an addition amount of Pd is 20000 mass ppm or less because “when the addition amount is more than 20000 mass ppm, hardness of a ball is increased and pad damage is caused at the time of ball bonding (refer to paragraph 0021 of Japanese Unexamined Patent Application Publication No. 2012-49198).
Moreover, such a wire for ball bonding is disclosed that contains a total of 5 to 500 wt. ppm of two or more elements chosen from Ca, Cu, Gd and Sm, a total of 0.5 to 5.0 wt. % of one or more element chosen from Pd and Au, and the balance being Ag and inevitable impurities (Japanese Unexamined Patent Application Publication No. 2012-151350). However, this bonding wire is a wire for bonding (W) that is used for connecting an Ni/Pd/Au coated electrode of a semiconductor device and conductor wiring of a circuit wiring board by a ball bonding method, and is not for joining a pad electrode of an Al alloy (Al—Si—Cu or the like). This is because “a joined portion of Al and Ag is likely to corrode” (refer to paragraph 0015 of Japanese Unexamined Patent Application Publication No. 2012-151350).